1. Field of the Invention
The present invention relates to an optical multi/demultiplexer device with loop-back optical paths having an arrayed waveguide grating, applicable to optical communication systems and optical switching systems. The device is simple in construction, and can be fabricated with high yield.
2. Technical Background
Conventionally, an optical add-drop multiplexer (referred to as ADM) such as the one shown in FIG. 19 is known as a key device for use in splitting and inserting wavelength-multiplexed optical signals. The ADM 1 comprises a demultiplexer 2, a multiplexer 3, and N lines of optical fibers 4a, 4b, . . . 4n.
In the optical ADM 1 circuit, multiplexed input optical signals consisting of wavelengths .lambda.1, .lambda.2, . . . , .lambda.n are separated into optical signals of N wavelengths from which desired optical signals, for example, .lambda.i and .lambda.j, are outputted. The remaining optical signals are transmitted through the optical fibers 4a, 4b, . . . 4n, which are multiplexed with the external signals .lambda.i, .lambda.j, and are outputted as multiplexed optical signals .lambda.1, .lambda.2, . . . , .lambda.n.
Another conventional ADM is shown in FIG. 20.
This ADM 5 is disposed between two optical transmission lines 6, 7, and comprises a demultiplexer 11, a multiplexer 12, 7 lines of optical fibers 13a, 13b, . . . 13g and a signal processing device 14 provided for each of the optical fibers 13a to 13g. In this case, seven wavelengths are shown for brevity, although in general, any number of wavelengths can be multiplexed.
In the above ADM 5, the multiplexed input optical signal of wavelengths .lambda.1, .lambda.2, . . . , .lambda.7 is first separated into optical signals of seven wavelengths by the demultiplexer 11, and then these optical signals are transmitted by the corresponding optical fibers 13a to 13g. The separated optical signals are processed by each of the signal processing device 14, are converted into electrical signals and are outputted from the ADM 5 to transmit the information forward. The response to the forwarded information or to a new piece of information is converted into an optical signal by the same signal processing device 14, and is inputted into a corresponding optical fiber 13. The optical signals transmitted through the optical fibers 13a to 13g are multiplexed by the multiplexer 12, and are outputted as multiplexed optical signals of wavelengths .lambda.1, .lambda.2, . . . , .lambda.7, and are forwarded to the optical line 7.
Further in this ADM 5, signal processing is carried out on all the wavelengths, but in general it is irregular to process all the signals. In such a case, for the wavelengths which need not be processed, only the optical fibers 13 are needed, and signal processing devices 14 can be omitted.
Also, there is known an optical delay line memory which delays pulsed optical signals and stores delayed optical pulses.
The optical delay line memory is classified into two large categories depending on the operational mode, into a tap type, represented typically by a parallel distribution type; and a loop type represented typically by a looping delay type.
FIG. 21 schematically illustrate the parallel distribution type optical delay line memory.
This optical delay line memory 21 comprises: a fixed wavelength light source 22; a 1.times.N optical coupler 23 which divides the optical pulses from the light source 22 into N optical paths; a plurality of delay fibers 24a, 24b, . . . , 24n which provide delay times i.tau. (i=1, 2, . . . , N); an N.times.1 optical switch 25 which selects one pulse of the delayed optical pulses given a delay of i.tau.; and an optical detector 26 which converts the optical pulses into electrical signals.
This optical delay line memory 21 has an advantage that the variations in the optical losses in a plurality of transmission lines are low.
FIG. 22, is a schematic illustration of the looping type delay line memory.
The optical delay line memory 31 comprises: a fixed wavelength light source 22; a 2.times.2 optical coupler 32; a delay line optical fiber 33 which constitutes a loop for propagating the signal; an optical amplifier 34; an optical switch 35; and an optical detector 26.
In the above optical delay line memory 31, the optical pulses forwarded from the fixed wavelength light source 22 are inputted into the loop containing the delay line optical fiber 33 through a 2.times.2 optical coupler 32. In this loop, when a pulse signal loops around i times around the loop, the delay time is given by i.tau. (where i=1, 2, . . . , N). The optical pulses having been delayed by the desired time duration, pass through the optical switch 35 by the gating action of the optical switch 35, and are converted into electrical signals by the optical detector 26. In this case, the intensity of the input optical pulses to the 2.times.2 optical coupler 32 decreases in principle by 1/4 every time the pulse loops through the coupler 32; therefore, when the pluses loop around N times, the intensity decreases to 1/2.sup.(N+1). An optical amplifier 34 is used to compensate for the loss in intensity.
The advantage of the optical delay line memory 31 is that the scale of the hardwares for propagating the signal around the loop is small.
In the meantime, an optical multi/demultiplexer having an arrayed waveguide grating type, shown in FIG. 23 has been proposed.
This optical multi/demultiplexer (referred to as a multi/demultiplexer hereinbelow) 41 is provided with N input waveguides 43, slab waveguides 44, 45 of depressed surface type, arrayed waveguide grating 46 and N lines of output waveguides 47, all of which are mounted on a substrate 42. Multiplexed input signals, of wavelengths constituted by .lambda.1, .lambda.2, . . . , .lambda.n, inputted into the input waveguide 43 are separated into N signals of wavelength .lambda.i and output them from the corresponding output waveguides 47j (j=a, b, . . . , n).
In the above ADM 1, both a demultiplexer 2 and a multiplexer 3 are used as a pair, therefore, it is necessary to precisely match the device characteristics of the demultiplexer 2 and the multiplexer 3. However, in practice, it is extremely difficult to manufacture such identically-matched devices, and their yield has been very poor. This was one of the reasons for a high cost of such optical devices.
In the other type of ADM 5 also, as in the above-mentioned ADM 1, it is necessary that the operating characteristics of the demultiplexer 11 and the multiplexer 12 be matched precisely. Therefore, such a system has a disadvantage that a paired device must be selected carefully from a production lot, thus leading to low production yield. The configuration of the ADM 5 also has a problem that it tended to be too large.
Also, because the above mentioned optical delay line memory 21 uses a 1.times.N optical coupler 23 and an N.times.1 optical switch 25, it is mandatory to have optical couplers and optical switches of uniform optical intensity loss and optical division ratio, thus leading to one major disadvantage that the number of the operating component parts required increases, and the number of steps in the joining operation increases. It follows, therefore, that the number of optical parts for making the system also increases, and the economics of the system suffers.
Further, as the number of division (N) increases, it becomes difficult, in particular, to fabricate N.times.1 optical switches 25 for varying the magnitude of optical delay times.
Further, in the above optical delay line memory 31, it is not possible to make a loop gain of 1, thus leading to the basic deficiency that the optical intensity loss increases as the optical pulses are propagated around the loop, and that the spontaneous emission noise accumulates leading to a degradation in the S/N ratio.
Further, in the above multi/demultiplexer 41 of the arrayed waveguide grating type, multiplexed optical signals consisting of .lambda.1, .lambda.2, . . . , .lambda.n are separated into N pieces of optical signals .lambda.i, and are outputted from the corresponding output waveguide 47j. However, there are many unused input waveguides 43 and output waveguides 47, and the utilization factor is low, thus wasting the vast multiplexing capabilities of this optical device.